Naegleria fowleri

(endorsed 1996)

Guideline

No guideline value is set for Naegleria fowleri in drinking water, but an ‘action level’ is recommended for water supplies likely to be contaminated. If the organism is detected, advice should be sought from the relevant health authority or drinking water regulator.

General description

Naegleria fowleri is a free-living, thermophilic amoeboflagellate which causes the waterborne disease primary amoebic meningoencephalitis (PAM). This rare but fatal condition has followed use of water for swimming, or domestic bathing. The organism occurs naturally in freshwater of suitable temperature, feeding on bacteria. Its occurrence is only indirectly related to human activity, inasmuch as such activity may modify temperatures or promote bacterial production. PAM has been reported from many countries, usually associated with thermally polluted environments, geothermal water or heated swimming pools. N. fowleri is almost exclusively aquatic, and water is the only known source of infection. Numerous nonvirulent Naegleria species are known in Australia.

Australian significance

PAM cases have been recorded from South Australia, Western Australia, Queensland and New South Wales; Naegleria fowleri has been detected in water in each of these states and in the Northern Territory. Australia is the only country where N. fowleri has been detected in public water supplies (Dorsch et al. 1983). Most of the available data on the density of N. fowleri in water relates to water supplies in South Australia (including the highest reported densities). In temperate Australia, significant seasonal cycles of density occur, from below one organism per litre to hundreds or thousands per litre in poorly disinfected water (Robinson and Christy 1984). N. fowleri detected at water temperatures below 18°C is likely to be present as cysts, which are not infectious, but which may seed a suitable environment.

Treatment of drinking water

Free chlorine or chloramines at 0.5 mg/L or higher will control N. fowleri, provided that the disinfectant persists throughout the water supply system. Chloramination is the preferred process in extensive rural water supplies, owing to its stability (Robinson and Christy 1984).

Method of identification and detection

Detection of amoebae, concentrated from water samples, requires relatively simple growth media and standard laboratory incubation facilities. Identification of Naegleria species, particularly recognition of N. fowleri, is more specialised. In routine or investigative analyses, presence of any thermophilic amoebae (able to grow at 42°C or above) is evidence that conditions are suitable for N. fowleri should it be introduced. If samples include any Naegleria, remedial action should be taken immediately without waiting for specific identification.

Prospective studies directed at water supplies that are susceptible to colonisation by N. fowleri can be valuable since the mortality rate of infection is so high, but universal monitoring is not appropriate.

Health considerations

N. fowleri is apparently an accidental pathogen. Its unusual route of infection (intranasal) means that PAM is associated with bathing rather than with ingesting water. Treatment is rarely effective, even in cases diagnosed early, and PAM is almost invariably fatal. Most Australian victims have been children (Dorsch et al 1983).

Recreational bathing presents the greatest risk of infection by N. fowleri, owing to the nature and duration of exposure, but domestic bathing can also lead to infection (Dorsch et al. 1983). Public water supplies can therefore be important as sources of contamination of public or private swimming pools, or as direct sources of infection. The infectious dose is unknown, but the frequency of infections has been low, even in populations that seem to have been widely and repeatedly exposed. A density of around 100 organisms per litre may present an immediate risk of infection but rapid density changes of this free-living organism can occur (Robinson and Christy 1984).

Derivation of guideline

No guideline value is proposed for N. fowleri, given its irregular distribution in Australia and its dependence on relatively high water temperatures. However, any water supply that seasonally exceeds 30°C or that continually exceeds 25°C can support the growth of N. fowleri. In such cases, a periodic prospective study would be valuable, but regular monitoring is not warranted unless N. fowleri is detected. A density of 2 organisms per litre (or detection in a 500 mL sample) is an appropriate threshold for action, given the rapid density changes that can occur. Other thermophilic Naegleria can be useful ‘proxy’ organisms for N. fowleri, allowing early remedial action.

References

Dorsch MM, Cameron AS and Robinson BS (1983). The epidemiology and control of primary amoebic meningoencephalitis with particular reference to South Australia. Transactions of the Royal Society of Tropical Medicine and Hygiene, 77, 372–377.

Robinson BS and Christy PE (1984). Disinfection of water for control of amoebae. Water, 11, 21–24.